Ukraine has ordered a full military mobilisation after what the country's president has described as Russia's "declaration of war."

Thousands of Russian troops have entered Crimea, seizing Ukrainian military bases and entering into a face-off with Ukrainian troops.

President Vladimir Putin declared he had the right to invade his neighbour in Moscow's biggest confrontation with the West since the Cold War.

"This is not a threat: this is actually the declaration of war to my country," Ukrainian Prime Minister Arseny Yatseniuk said in English.

Yatsenuik heads a pro-Western government that took power in the former Soviet republic when its Moscow-backed president, Viktor Yanukovich, was ousted last week.

Putin secured permission from his parliament on Saturday to use military force to protect Russian citizens in Ukraineand told U.S. President Barack Obama he had the right to defend Russian interests and nationals, spurning Western pleas not to intervene.

Furious American officials vowed that President Putin would pay a heavy price through economic and diplomatic sanctions.

America and Britain yesterday called on Russia to end its so-far bloodless military intervention.

US Secretary of State John Kerry warned that it risked being booted out of the G8 and becoming an international pariah.

He added: “The G8 plus some others, every single one of them, are prepared to go to the hilt in order to isolate Russia.”

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Foreign Secretary William Hague flew to the Ukrainian capital Kiev yesterday as experts warned the crisis could push up gas prices in Britain – as has happened in previous conflicts involving Russia.

The country provides about a quarter of Europe’s gas and around 12% of that used in Britain, with Ukraine the main supply route.

And that could prove a powerful lever, one observer claimed last night. Leslie Holmes, professor of political science at the University of Melbourne, said: “Ukraine is still very reliant on energy from Russia and Europe is heavily dependent on Russian oil and gas.

“That means Russia still has a trump card up its sleeve.”

As demonstrations against the Russian incursion were staged in Moscow, Kiev and London, Barack Obama and David Cameron discussed Russia’s “unacceptable” actions by phone last night.

Mr Cameron said he and the US President agreed that Moscow must face “significant costs” if it did not withdraw military forces.

Britain has already announced a boycott of G8 talks due to take place at the Winter Olympics site in Sochi.

And it was revealed yesterday that ministers will also stay away from the Paralympics at the Black Sea resort.

The Ukraine crisis was triggered after the pro-Moscow government in Kiev was toppled. Russian troops have now moved in to Crimea, taking over key installations and government offices.

Russian PM Dmitry Medvedev said Ukraine’s new leaders had seized power illegally. He added chillingly: “It will end in a new revolution. New blood.”

Ex-Lib Dem leader and special forces veteran Paddy Ashdown, a former Balkans envoy, warned: “We are one pace away from catastrophe. It would require just one foolish act, one trigger-happy soldier, to tip us over the edge.”

Mr Putin defended his actions in a phone call with German Chancellor Angela Merkel. He insisted he was protecting his people against “ultra- nationalist forces”.

But Ms Merkel accused him of breaching international law with an “unacceptable intervention”